What happened?
A software error has been detected on the MMU unit.
How to fix it?
Make sure that you are using the correct up-to-date firmware for both the printer and the MMU unit: Firmware download.
If that does not solve the error, take note of all the actions that preceded the error happening. Try to reproduce the error, and save the log.
After that, contact our Zákaznická podpora, or report the issue directly in the Prusa GitHub page.